1. Field of the Invention
The present invention relates to a stepper motor having solenoid coils around end portions of stator poles, and in particular to a stepper motor construction that has solenoid coils axially mounted on two ends of the stator yoke to strengthen the magnetic flux intensity.
2. The Related Art
A stepper motor converts electric pulses into incremental mechanical motion. Because of the precision control of machine operation, the stepper motor has wide industrial applications.
FIG. 1 of the attached drawings illustrates a known stepper motor, which comprises a rotor 2 with N pairs of poles on a radially disposed permanent magnet from the core, at least two W-shaped yokes 11, 12 encompassing the rotor 2, each of which has a coil winding 110/120 axially wound around a central strut 111/121. The central strut 111 is equal distance from pole shoes 112, 113, and the central strut 121 is equal distance from pole shoes 122, 123, forming stator pole groups.
When the coil winding 110 is energized, electrical current flows through the coil wound around the central strut 111 to produce a N pole, and the magnetic flux cuts through two adjacent shoes 112, 113 on the silicon steel yoke 11 to give two S poles. These stator poles attract respective poles on the rotor 2 to hold the rotor in step position. However, when the direction of current flow through the coil is changed, the central strut 111 becomes S Pole, and the pole shoes 112, 113 become N poles, so the previously attracted poles of the rotor 2 are now repelled. The coil windings 110, 120 are energized in accordance with a phase switching sequence. This switching magnetic field therefore produces a movement force to propel the rotor 2 into rotation.
The pole group on the yoke 11/12 is to be matched by respective pole group on the rotor 2, which means the rotor pole group has to be within the reaction area to attract or repel respective pole group on the rotor 2. It shall be noted that the divided members of the pole groups on the rotor 2 are separated by a step angle.
When the central strut 111 on the yoke 11 is attracted by respective pole on the rotor 2, two adjacent poles 112, 113 on the right and left sides of the central strut 111 are also lined up with respective poles on the rotor 2 having opposite polarity.
When the rotor 2 is held in step position, the central strut 111 and two pole shoes 112, 113 on the yoke 11 are respectively attracted to poles on the rotor 2, and at the same time, the central strut 121 and two pole shoes 122, 123 on the yoke 12 are not aligned with any rotor pole, because the pole groups are offset by control buffers of appropriate magnetic pole pitch.
Referring to FIG. 2 of the attached drawings, an ideal path of a magnetic flux through the rotor 2 and yoke 11 is shown. The ideal magnetic flux indicated by the bold broken line begins from one end of the central strut 111 crossing the air gap to an opposing pole on the rotor 2, and then passing to an adjacent pole of opposite polarity before turning back to pole shoe 112/113 on the yoke 11, and then cutting through the yoke 11 to return to the central strut 111 from the other end, thus completing the magnetic flux into a closed loop. Since a large part of the magnetic flux travels through magnetic material and encircles with smaller radius, the distribution of magnetic flux encounters low magnetic reluctance, thus avoiding magnetic loss or magnetic flux leakage. Magnetic reluctance is related to resistance encountered in the magnetic flux distribution.
Again referring to FIG. 2, the actual distribution of a typical magnetic flux through the rotor 2 and yoke 11 is shown by the dotted line. The distribution path starts from one end of the central strut 111 same as that in the previous example, cutting across to an opposing pole on the rotor 2, and then passing through an adjacent pole of opposite polarity, and then turning around to a pole shoe 112/113 on the yoke 11, and then passing through the air gap before returning to the central strut 111 from the other end. Since the magnetic flux is distributed in a radial pattern from the central strut, and the two adjacent pole shoes are bare iron, a large part of the magnetic flux is distributed through the air media, not the magnetic material (solenoid coils and yoke), so the magnetic field strength is considerably weakened in the last part of the distribution.
Furthermore, because of the distance between the central strut 111 and the pole shoes 112, 113 on two sides, when the magnetic flux circles around the pole shoe 112/113 beyond the perimeter of the yoke, the magnetic field strength is drastically lessened.
For an ideal motor design, the structure of the circumferential stators shall be strong enough to protect the inside components, but the current designs often have 1 cm lamination on the stator yoke, and the distance between the central strut 111 and the two pole shoes 112, 113 are reduced to 2-4 mm. In such situation, to have more than one hundred turns of coil wound around the central strut, high level skill on the part of the assembly workers is required. This stepper motor construction is therefore not suited for miniature motors, and the yield rate is hardly improved using the labor-intensive winding process.
Referring to FIG. 3, another conventional stepper motor of the prior art has a rotor 3 having multiple poles and two yokes 41, 42 on the circumference. The first yoke 41 on the circumference has two end portions 411, 412 to give a pole pair, and the two poles 411, 412 are connected by a coil winding 410 with bobbin 413, where the second yoke 42 is divided into two end portions 421 and 422 by the winding 420 and bobbin 423.
Referring now to the first stator structure shown in FIG. 4, the A portion represents the position of solenoid coil on the stator yoke, linking the two end portions C of the yoke. Then in the second structure shown in FIG. 5, two coil windings are disposed on the C portions.
According to the empirical design, a formula is established to calculate the coefficient of magnetic loss in the distribution of magnetic flux from the first structure shown in FIG. 4:f=1+(Lg/Ag){[1.7Ua×a/(a+Lg)]+[1.4b(Ub/c)1/2+0.67Uc]}  (1)where Lg represents the length of the air gap, Ag represents the cross-sectional area of the air gap, and Ua, Ub, and Uc respectively represent the circumferences of the A, B and C portions.
Assuming the cross-sectional area of the air gap and A, B, and C portions of the first structure equal to ½ cm×½ cm, and the lengths of the A portion and air gap are 1 cm, and lengths of the B and C portions are 3 cm, then these data are plugged into Equation 1 to produce the magnetic loss as follows:f=1+{(1.7×4×½)+[1.4×3( 4/3)1/2+0.67×4]}=1+10.93=11.93
On the contrary, the magnetic loss from the second structure shown in FIG. 5 is derived from the following equation:f=1+(Lg/Ag)×1.1Ua×{[0.67a/(0.67a+Lg)]+(Lg/2a)}  (2)
Using the same conditions as the first structure to plug into Equation 2:f=1+4.4(⅖+½)=1+3.96=4.96
If the initial magnetic flux on the two structures is 100%, the remaining magnetic flux intensity after passing through the air gap of the first structure is only 8.38% of the initial value; contrarily, the effective magnetic flux intensity through the air gap in the second structure is 20.14%, which is considerably more than that through the first structure.
From these two examples, although magnetic loss or magnetic flux leakage through the poles and yoke in the two-stator structures is inevitable, the result can be quite different if the coil windings are careful arranged on the stator.
Also, it is obvious that the stator yokes 411 and 412 shown in FIG. 3 are not of equal length, so the resulting magnetic flux through the two stator poles are unevenly distributed. The magnetic flux intensity would be adversely affected.
The present invention is aimed to provide a stepper motor with solenoid coils around stator poles which possesses many advantages over the conventional stepper motor.